A large body of evidence exists on the role of dopamine in reinforcement learning. Less is known about how dopamine shapes the relative impact of positive and negative outcomes to guide value-based choices. We combined administration of the dopamine D2 receptor antagonist amisulpride with functional magnetic resonance imaging in healthy human volunteers. Amisulpride did not affect initial reinforcement learning. However, in a later transfer phase that involved novel choice situations requiring decisions between two symbols based on their previously learned values, amisulpride improved participants' ability to select the better of two highly rewarding options, while it had no effect on choices between two very poor options. During the learning phase, activity in the striatum encoded a reward prediction error. In the transfer phase, in the absence of any outcome, ventromedial prefrontal cortex (vmPFC) continually tracked the learned value of the available options on each trial. Both striatal prediction error coding and tracking of learned value in the vmPFC were predictive of subjects' choice performance in the transfer phase, and both were enhanced under amisulpride. These findings show that dopamine-dependent mechanisms enhance reinforcement learning signals in the striatum and sharpen representations of associative values in prefrontal cortex that are used to guide reinforcement-based decisions.